Mechanisms of Conductive Presbycusis In Humans Age-related high frequency hearing loss (presbycusis) is a near universally experienced condition, affecting tens of millions of individuals in the United States alone and costing worldwide more than $750 billion per year. Presbycusis results in difficulty understanding speech, problems hearing environmental sounds and, if unaddressed, can lead to social isolation. Given current demographic trends, the societal burden of presbycusis is expected to accelerate. Despite the prevalence of presbycusis, much remains unknown about its etiologic mechanisms. Presbycusis has long been assumed to be secondary to sensorineural dysfunction, but emerging evidence suggests conductive pathology contributes to presbycusis. The principal method for clinical evaluation of hearing is behavioral pure tone (500-8kHz) audiometry. Extended high frequency air conduction testing (>8kHz) is now commonly performed given new knowledge on the role of high frequency sound for speech understanding and sound localization. Unfortunately, clinical bone conduction testing stops at 4kHz. Above 4kHz, limitations in standard stimulus transducers that are largely unchanged from the 1950s, and a lack of normative bone conduction standards limit study of high frequency conductive loss. Differentiation between sensorineural and conductive presbycusis is not routinely performed, but is of interest because: 1) prior work demonstrates discrete changes within the middle ear can lead to isolated high frequency conductive loss (ie: with normal low-frequency thresholds) and 2) our ability to surgically repair the middle ear. Further investigation into the prevalence and functional significance of age- related middle ear change is necessary to ensure emerging therapies for presbycusis are appropriately directed. Newly developed bone conduction transducers without high frequency limitations permit comprehensive bone conduction testing at frequencies up to 16kHz. Novel transducers will help to finally establish whether a clinically relevant conductive or mixed presbycusis exists, shedding light on the functional effects of the aging middle ear. Development of reliable testing for high frequency conductive hearing loss carries implications beyond presbycusis, including post-middle-ear-surgery hearing assessment. Our goal is to identify mechanisms of conductive presbycusis and to establish methods to reliably diagnose high frequency conductive hearing loss. We hypothesize that a subset of patients with presbycusis have increased ossicular compliance resulting in conductive or mixed high frequency hearing loss.